Rate aided tracking for launch transient survivability

ABSTRACT

Rate aiding tracking prevents or dampens the effect of large scale target tion on a missile borne tracker during transition of a missile from prelaunch tracking to post launch tracking. Rate aided tracking includes a feedback network responsive to existing stable platforms for combining a tracker output signal with a rate signal that is itself responsive to torques present on a stable platform due to target sensor motion. The feedback network includes an integrator and a transformation circuit and is active only during the missile launching phase of operation when transient effects are manifest.

DEDICATORY CLAUSE

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto me of any royalties thereon.

BACKGROUND OF THE INVENTION

Maintaining target track during missile launch transient perturbationsis a long standing problem for lock-on-before-launch (LOBL) targettracking seekers. The launch transient must be accomodated by the seekerwhen the target range is unusually large; consequently, targetsignal-to-noise (S/N) and apparent target size are at their minimum.Such conditions severely limit the ability of conventional targettrackers to follow any large target motion resulting from a launchtransient induced sensor rotation. A prior art solution of the launchtransient problem has been to increase the performance capability of thesensor stabilization platform to achieve the degree of missile airframeto sensor isolation which limits to an acceptable level the amount oftarget motion that must be tracked. However, for high performanceairframes the degree of stabilization performance required becomes amajor cost driver for LOBL seekers.

SUMMARY OF THE INVENTION

For rate aided tracking, pitch and yaw rate information and missile rollrate information are used by the tracker to predict target motion duringthe launch transient. This auxillary information is used to electricallyreposition the target track gate and thereby compensate for any targetmotion that results from sensor rotation during the launch period. Thisreduces the work load of the tracker from that of tracking a perceivedlarge target motion to that of tracking the difference between the largemotion and an estimate of that motion. This tracking method increasesthe ability of the target tracker to maintain lock-on during targetpertubations induced by undesirable platform motion before launch.

Rate aided tracking is applicable to any LOBL seeker that uses a ratestabilized sensor platform and a tracker with a capability of beingelectronically re-aimed. The advantage of rate aided tracking is thatseeker stabilization performance requirements can be greatly reducedbecause of the increased capability of the traget tracker. The decreaseof the seeker stabilization performance also reduces cost.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a block diagram of a preferred embodiment of thesystem for providing platform rate information to the tracker.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, the rate aiding circuitry 10 is shown indashed lines and is coupled with a standard target tracker 26 and torate stabilized platform and associated circuitry. A typical ratestabilized platform will involve two degrees of freedom, i.e., it willrespond to changes with reference to two coordinate axes normallyreferred to as x for yaw and y for pitch. It does not respond to changesin the third axis normally referred to as z for roll. The single FIGUREshows a stable platform 4 and related circuitry for providing x and ycoordinate axis processing. The y axis processing 6 is shown generallyand is identical to the x axis processing discussed in detail.

In a tracking system as shown generally in the single FIGURE, missilemotion results in forces being applied to components and systems withinthe missile housing. Stable platforms such as platform 4 are sensitiveto these motions. Thus, when missile motion forces are applied duringthe transition of a missile from a stationary position within a launcherto unemcumbered flight position free of the launcher, transient forcesaffect the missile and stable platform therein. These transient forcesare manifest in the inertia of the platform, in the gimbal springs, andthrough friction, to cause a physical or mechanical misalignment of atarget sensor 24 from the electro-optically established target LOS andof the rate sensor 34. A torque motor 36 is coupled to the stableplatform to respond to forces on the platform to induce a stabilizinginfluence on the platform. The torque motor can also provide anoffsetting or neutralizing response to undersirable motion. In thedrawing, these correctional forces are depicted as a mechanical force indashed lines 33 applied from the torque motor into the target sensor 24and to rate sensor 34.

In the system target sensor 24, normally composed of an opticaltelescope system and a detector, converts an impending target scene intoan electrical signal 25. The x and y coordinate position of the targetwithin this electrical signal is determined by the angular displacementθ of the optical axis of the sensor 24 with respect to the inertialline-of-sight (LOS) between the target position and the tracking systemreference position or optical axis. The sensor output electrical signal25 is operated upon by the tracker 26 to extract the target's xcoordinate 28. The tracker output 28 is multiplied by a velocityconstant in multiplier 30 to produce an output rate command 32 that iscoupled back to the stable platform.

Stable platform 4 includes the rate sensor 34, target sensor 24, andtorque motor 36. Other circuitry may be on or attached to the platformbut are not pertinent to the operation. Missile body motion producesdisturbance torque 63 on the stable platform resulting from normalmechanical gimbal spring and friction coupling. Spring and frictioncoupling is made as small as possible but in a practical system it cannever be completely eliminated. Summing circuit 44 receives the outputrate command 32 and also receives a feedback signal 35, providing anoutput which is coupled to the gain and compensation circuitry 40. Thegain and compensation circuitry 40 contains standard state of the artcomponents for providing stable closed loop operation of the rateplatform control loop. The electrical output of circuit 40 is convertedto a mechanical torque 37 by torque motor 36.

The platform 4 responds to the total torque applied to it by moving at arate determined by the magnitude of the applied torque and the inertiaof the platform. However, rate sensor 34 is fixed to the stabilizedplatform and thus responds to the motion of the platform by generatingan electrical output proportional to the platform rate, which providesthe feedback input 35 to summing circuit 44. As the stable platformmoves at a rate in response to the applied undesirable torque aninertial platform angle is developed and, since the target sensor 24 isfixed to the stable platform, the difference between this platform angleand the target line of sight angle is measured by the target sensor 24.The rate aiding circuitry 10 is shown in dashed lines and comprises acoordinate transformation circuit 11, and integrator 12, and a disablecircuit 13 coupled in series with a summing circuit 14. Coordinatetransformation circuit 11 also receives the platform rate gyro output35. In the general case this transformation is the well known EulerEquation and has input angles and rates from all three coordinate axis.In the case shown for only one axis of processing this transformationreduces to unity. The transformation circuit is followed by integrator12 which integrates the transformer signal to provide output 54 which isan estimate of the platform 4 motion. The integrator 12 is in serieswith disable circuit 13. During the period of launch disable circuit 13has a gain of 1. Summing circuit 14 receives the output of the disablecircuit 13 and further receives the output 28 from tracker 26. These twosignals, combined, are coupled 58 into a summing circuit 60 and combinedwith the target sensor 24 output for coupling to tracker 26. Thiscombined signal 58 allows the position of the tracker gate with respectto the target sensor field-of-view (FOV) to be shifted in directproportion to an external electronic command. The electronic command isthe output of integrator 12. In this manner the track gate is moved inthe same direction as the target motion in the sensor FOV due to sensorrotation. This controlled or directed tracker movement is independent ofthe ability of the tracker to follow target motion and is used tocompensate for missile motion forces during the transient phase ofmissile launch. However, once the launch is accomplished the gain ofdisable circuit 13 is reduced to zero in a smooth manner, using a timeof approximately 0.5 seconds to go from a gain of 1.0 to zero,eliminating any output as the gain goes to zero. This allows normalcontrol to be reestablished via 34, 35, 32, and 44.

The exact method of moving track gates in response to an externalcommand varies with the particular type of tracker being used. Theparticular method used to move a track gate is important only for therequirement that the gate motion must be accomplished withoutinterrupting the basic tracking function. For clarity, only a planer(single-axis) rate aided modification is shown. The coordinate data forthe other axis must also be provided to the tracker, as shown generallyin the drawing. Alternatively, the simple, single axis, rate integration10 can be replaced by a two or three axis signal integration using theappropriate and well known Euler equations. In addition, the missileroll rate sensor and the platform pitch and yaw rate sensors (not shown)can be used so that roll transients can be accommodated. It should benoted that the success of this additional approach requires an accuratemeasure by these rate sensors during the period of the launch transient,i.e., the period from ignition or firing of the rocket motor until themissile is free and clear of the launcher. During missile launch severemissile motion and acceleration create the disturbance torques uponplatform 4 which act to perturb the normal platform rate and platformposition information. In normal operation as a closed loop tracker/rateplatform form, the rate platform loop consisting of the rate gyro orrate sensor 34, torque motor 36 and platform gain and compensation 40offset these perturbations, operating to provide a stable inertialreference on which sensor 24 is mounted.

During platform perturbation, the platform rate gyro (sensor 34)produces output 35 while the rate platform loop (44, 40, 36) responds toany disturbance torques from missile forces (63). The output response 35is integrated by the electronic integrator 12 to produce the estimate ofplatform position. The rate is integrated by integrator 12 to provide aplatform 4 position which is subtracted geometrically from the detectedtarget LOS angle θ and results in sensor 24 measuring a target position(output 25) equal to this difference.

Alternatively, for a three dimensional case a set of Euler equationsbased on missile roll rate as well as platform pitch and yaw rates maybe integrated to form the estimated platform position. This estimate canthen be input to a summing circuit, which functions as the electronicgate position shifter. The shifting of the track gate position (input27) is equivalent to a subtraction at summing circuit 60 of theestimated platform position from the sensor measured target position 25.Thus, tracker 26 needs respond only to the target LOS angle θ plus thedifference between the estimated motion of platform 4 (due to launchtransient) and the actual motion. The operation of the remainder of theloop is straight forward. The tracker output 28 is multiplied by theseeker velocity constant 30 to produce the platform rate command 32.Command 32 is then input to the rate platform summing junction 44 toclose the track loop. After the period of the launch transient,integrator 12 cannot be allowed to remain in the loop and must bedischarged to zero at a rate trackable by the tracker 26 as hereinabovenoted.

Although the present invention has been described with reference to apreferred embodiment, workers skilled in the art will recognize thatchanges may be made in the form and detail without departing from thescope and spirit of the foregoing disclosure. Accordingly, the scope ofthe invention should be limited only by the claims appended hereto.

I claim:
 1. In a target tracking system wherein a target sensor on amissile is directed by a tracker to maintain track of a target positionby summing the line-of-sight deviation of the target from the referenceaxis of a stable platform on the missile, the improvement comprising: arate aiding circuit coupled between the stable platform and the trackerfor stabilizing the tracker input signals during the missile launchingtransition period, said rate aiding circuit having at least first andsecond inputs and an output for receiving input signals from the stableplatform and from the tracker respectively and providing a maximumoutput signal to the tracker during the launching transition period anda minimum output signal thereafter.
 2. In a target tracking system asset forth in claim 1 said rate aiding circuit comprising: an integratorcoupled to receive a signal indicative of changes occurring in stableplatform position and for providing an integrated signal output, adisable circuit having an input coupled to the integrator output forproviding a maximum gain output; and a summing circuit having first andsecond inputs and an output, said first input being coupled to theoutput of said disable circuit, said second input being coupled toreceive the output from said tracker, and said output being coupled toprovide an input to said tracker during missile launch.